The objective of this study was to evaluate the suitability of steel slag as an additive to the engineering properties of weak clay soil. Different geotechnical laboratory tests were conducted on both stabilized and natural soils. Steel slag (SS) was added at a rate of 0, 5, 10, 15, 20, and 25% to the soil. Specific gravity, grain size analysis, Atterberg limit test, compaction test, free swell, California bearing ratio (CBR), and unconfined compression strength (UCS) are among the tests that were performed. The Atterberg limit test result shows that the liquid limit decreases from 90.8 to 65.2%, the plastic limit decreases from 60.3 to 42.5%, and the plasticity index decreases from 30.5 to 22.7% as the steel slag of 25% was added to the expansive soil. With 25% steel slag content, the specific gravity increases from 2.67 to 3.05. The free swell value decreased from 104.6 to 58.2%. In the Standard Proctor compaction test, the maximum dry density rises from 1.504 to 1.692 g/cm3, while optimum moisture content falls from 19.77 to 12.09%. From the UCS test, mixing 25% steel slag into the soil increases the unconfined compressive strength from 64.3 to 170.6 kPa. Additionally, the CBR value increases from 3.64 to 6.82% as 25% of steel slag is mixed with the soil. As a result, steel slag has been found to improve expansive soil properties for geotechnical applications.